Real-Time TDM-Guided Optimal Joint PK/PD Target Attainment of Continuous Infusion Piperacillin–Tazobactam Monotherapy Is an Effective Carbapenem-Sparing Strategy for Treating Non-Severe ESBL-Producing Enterobacterales Secondary Bloodstream Infections: Findings from a Prospective Pilot Study

(1) Objectives: To assess the impact of optimal joint pharmacokinetic/pharmacodynamic (PK/PD) target attainment of continuous infusion (CI) piperacillin–tazobactam monotherapy on the microbiological outcome of documented ESBL-producing Enterobacterlaes secondary bloodstream infections (BSIs). (2) Methods: Patients hospitalized in the period January 2022–October 2023, having a documented secondary BSI caused by ESBL-producing Enterobacterales, and being eligible for definitive targeted CI piperacillin–tazobactam monotherapy according to specific pre-defined inclusion criteria (i.e., absence of septic shock at onset; favorable clinical evolution in the first 48 h after starting treatment; low–intermediate risk primary infection source) were prospectively enrolled. A real-time therapeutic drug monitoring (TDM)-guided expert clinical pharmacological advice (ECPA) program was adopted for optimizing (PK/PD) target attainment of CI piperacillin–tazobactam monotherapy. Steady-state plasma concentrations (Css) of both piperacillin and tazobactam were measured, and the free fractions (f) were calculated based on theoretical protein binding. The joint PK/PD target attainment was considered optimal whenever the piperacillin fCss/MIC ratio was >4 and the tazobactam fCss/target concentration (CT) ratio was >1 (quasi-optimal or suboptimal if only one or neither of the two thresholds were achieved, respectively). Univariate analysis was carried out for assessing variables potentially associated with failure in achieving the optimal joint PK/PD target of piperacillin–tazobactam and microbiological eradication. (3) Results: Overall, 35 patients (median age 79 years; male 51.4%) were prospectively included. Secondary BSIs resulted from urinary tract infections as a primary source in 77.2% of cases. The joint PK/PD target attainment was optimal in as many as 97.1% of patients (34/35). Microbiological eradication occurred in 91.4% of cases (32/35). Attaining the quasi-optimal/suboptimal joint PK/PD target of CI piperacillin–tazobactam showed a trend toward a higher risk of microbiological failure (33.3% vs. 0.0%; p = 0.08) (4) Conclusions: Real-time TDM-guided optimal joint PK/PD target attainment of CI piperacillin–tazobactam monotherapy may represent a valuable and effective carbapenem-sparing strategy when dealing with non-severe ESBL-producing Enterobacterales secondary BSIs.


Introduction
Infections caused by extended-spectrum beta-lactamase (ESBL)-producing Enterobacterales represent a global health concern.Several epidemiological studies have shown that ESBL-producing Enterobacterales resistant to third-generation cephalosporins may represent up to 35% and 18% of Klebsiella pneumoniae and Escherichia coli clinical isolates, respectively [1][2][3][4].In a recent meta-analysis, infections caused by ESBL-producing Enterobacterales were associated with higher mortality rates compared with those caused by non-ESBL-producing Enterobacterales [5].
In the MERINO trial, treatment of bloodstream infections (BSIs) caused by ceftriaxoneresistant Escherichia coli or Klebsiella pneumoniae with piperacillin-tazobactam did not reach a non-inferior mortality rate compared with meropenem [6].However, a large debate still exists nowadays about which therapeutic choices should be preferred in this scenario [7][8][9][10][11][12], as argued both in the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) guidelines and in the Infectious Diseases Society of America (IDSA) guidance [13,14].On the one hand, in the MERINO trial, findings of the inferiority of the piperacillin-tazobactam arm could have been affected by using an intermittent infusion dosing scheme of 4.5 g every 6 h over 30 min [6], which could have been suboptimal, also considering that in a post hoc analysis several clinical isolates were found to be resistant to piperacillin-tazobactam at broth microdilution testing [15].On the other hand, the worrisome ever-growing increase in carbapenem-resistant Gramnegative infections promoted by the selective pressure deriving from extensive carbapenem use [16,17] may call into question the potential role that carbapenem-sparing strategies based on piperacillin-tazobactam could have in some non-severe clinical scenarios of ESBL-producing Enterobacterales infections [10].In this latter regard, several well-designed observational studies have shown that no significant difference exists in terms of the mortality rate between piperacillin-tazobactam and carbapenems in the treatment of secondary BSIs caused by ESBL-producing Enterobacterales [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37].Additionally, the use of piperacillin-tazobactam compared with that of carbapenems was associated with a lower occurrence of colonization and/or infection caused by multidrug-resistant (MDR) or carbapenem-resistant Gram-negative pathogens.In a retrospective observational multicentric study including 151 patients with ESBL-producing BSIs, Ng et al. found that piperacillin-tazobactam was associated with a significantly lower acquisition of MDR bacterial infections compared with carbapenems (7.4% vs. 24.6%;p = 0.01) [20].Similarly, in a retrospective observational multicentric study including 186 patients affected by ESBLproducing bacteremic urinary infections, Sharara et al. reported a trend toward a lower rate of 30-day colonization, with carbapenem-producing Enterobacterales among those receiving piperacillin-tazobactam compared with those receiving carbapenems (2% vs. 8%; p = 0.09) [30].
Some studies showed that when using beta-lactam/beta-lactamase inhibitor combinations (BL/BLIc), attaining optimal joint pharmacokinetic/pharmacodynamic (PK/PD) targets of both the BL and the BLI may be beneficial in terms of both maximizing the clinical/microbiological outcome and preventing resistance development [38,39].In this scenario, implementing a real-time therapeutic drug monitoring (TDM)-based expert clinical pharmacological advice (ECPA) program may represent a valuable approach for assessing the optimal joint PK/PD target attainment of BL/BLIc administered by continuous infusion (CI) [40].
The aim of this study was to assess whether the optimal joint PK/PD target attainment of CI piperacillin-tazobactam monotherapy could represent a valuable carbapenem-sparing strategy in treating patients with documented BSIs caused by ESBL-producing Escherichia coli or Klebsiella pneumoniae.

Study Design and Inclusion Criteria
This prospective study was carried out between 1 January 2022 and 30 October 2023 in the medical wards, surgical wards, or intensive care units (ICUs) of the IRCCS Azienda Ospedaliero-Universitaria of Bologna, Italy.The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the local ethical committee (No.EM 232-2022_308/2021/Oss/AOUBo on 16 March 2022 and No. 894/2021/Oss/AOUBo on 15 November 2021).Signed informed consent was collected from each included patient.Patients were enrolled if they met the following inclusion criteria: (a) having a documented piperacillin-tazobactam fully susceptible ESBL-producing Enterobacterales secondary BSI (phenotypically identified as being resistant to ceftriaxone and/or cefotaxime, as previously reported [6]); and (b) being eligible for definitive CI piperacillin-tazobactam targeted monotherapy according to the following pre-defined inclusion criteria: absence of septic shock at onset; favorable clinical evolution during the first 48 h after starting empirical treatment with piperacillin-tazobactam; low-intermediate risk primary infection source, namely, urinary tract infection (UTI), biliary/intrabdominal infection (IAI), or catheterrelated (CR) BSI.Table 1 summarizes the inclusion criteria.All of the patients received piperacillin-tazobactam therapy optimized by means of a real-time TDM-guided ECPA program and at least one follow-up blood culture a minimum of after 48 h from starting therapy for assessing microbiological outcome.

Data Collection
Demographic data (age, sex, weight, height, and body mass index (BMI)), clinical/laboratory data (admission ward, Charlson Comorbidity Index (CCI), immunocompetence status, baseline creatinine clearance (CLCr), need for intermittent hemodialysis (IHD) or continuous renal replacement therapy (CRRT), occurrence of augmented renal clearance (ARC), and status of source control), microbiological data (type/site of infection, and ESBL-producing clinical isolate with an MIC value for piperacillin-tazobactam), piperacillin-tazobactam treatment data (dosing regimen at baseline, average piperacillin and tazobactam steady-state concentrations (C ss ) during treatment, number of TDM-based ECPAs, recommended dosing adjustments at first and at subsequent ECPAs, and treatment duration), and microbiological/clinical outcome data (microbiological eradication/failure, eventual resistance development, 30-day relapse, clinical cure, and 30-day mortality rate) were prospectively collected.
Immunocompetence status was defined as depressed whenever one or more of the following conditions existed: need for long-term use of corticosteroids and/or of biologic and/or antineoplastic agents; occurrence of solid or hematologic malignancies; previous solid organ (SOT) or hematopoietic stem cell transplantation (HSCT); and underlying HIV disease or autoimmune disease [41].
ARC was defined as a measured (based on 24 h urine collection) or an estimated (according to the CDK-EPI formula) creatinine clearance above 130 mL/min and 120 mL/min in males and females, respectively [42].
The status of source control was defined as failed whenever blood cultures were still positive seven days after the index culture, as previously reported [43].
The Centers for Disease Control and Prevention (CDC) criteria were adopted for defining the different types of infection [44].Secondary BSIs were defined on the basis of [44] the simultaneous isolation of the same pathogen from at least one blood culture drawn by direct venipuncture and from the primary source, namely, from the peritoneal fluid, abdominal specimens, or bile in the case of IAI [45,46]; from urine cultures with a bacterial load of at least >10 5 CFU/mL in the case of UTIs [44,45]; and from blood cultures drawn through a vascular device having 2 h shorter positivization time in the case of CR-BSI [44].

Definition of Outcome Variables
Microbiological outcomes were defined as eradication whenever the index pathogen was undetectable at the follow-up blood cultures, and, in contrast, as failure whenever the index pathogen was still detectable at the follow-up blood cultures (breakthrough BSI).
Resistance development was defined as an increase in the piperacillin-tazobactam MIC beyond the EUCAST clinical breakpoint of susceptibility.
Thirty-day relapse was defined as the re-growth of the index pathogen in blood cultures carried out within 30 days from stopping piperacillin-tazobactam therapy.
Clinical outcomes were defined as cures if a complete resolution of biochemical and clinical signs and symptoms of the infection was coupled with a documented microbiological eradication at the end of treatment and with an absence of relapse at the 30-day follow-up [49].
2.4.Piperacillin-Tazobactam Dosing Regimens, Sampling Procedure, and Implementation of a Real-Time TDM-Guided ECPA Program for Dosing Personalization Piperacillin-tazobactam was started with a loading dose of 9 g administered over 2 h infusion immediately followed by a maintenance administered by CI [50] that was initially chosen according to the status of the patient's renal function.
TDM of piperacillin and of tazobactam was assessed at steady-state (C ss ), firstly after at least 24 h from starting therapy in order to be in steady-state conditions, and subsequently whenever feasible every 48-72 h during the whole treatment course.Total piperacillin and tazobactam C ss were measured by means of a validated liquid chromatography-tandem mass spectrometry method [51].By considering the plasma protein binding of piperacillin and of tazobactam reported in the literature, namely, 20% and 23%, respectively [52], the free (f ) C ss were calculated by multiplying the total C ss by 0.80 and 0.77, respectively.
A real-time TDM-guided ECPA program supported by skilled MD Clinical Pharmacologists was used for optimizing the joint piperacillin-tazobactam PK/PD target in each individual patient, as previously reported [40].

Definition of Optimal, Quasi-Optimal, and Suboptimal Joint PK/PD Target Attainment of Piperacillin-Tazobactam
The PD determinant selected for assessing the efficacy of piperacillin-tazobactam monotherapy was a joint PK/PD target, as previously described [38].Briefly, the joint PK/PD target was considered optimal whenever the piperacillin f C ss /MIC ratio was >4 and the tazobactam f C ss /target concentration (C T ) ratio was >1 (where C T is the fixed tazobactam target concentration of 4 mg/L proposed by the EUCAST for testing the in vitro susceptibility of the piperacillin-tazobactam combination); it was considered quasi-optimal whenever only one of the two thresholds was attained and suboptimal whenever none of the two was attained [38].
In patients having multiple TDM-guided ECPA programs during the treatment course, the average piperacillin and tazobactam f C ss were calculated.The impact of the quality of the joint PK/PD target attainment of CI piperacillin-tazobactam on the microbiological outcome was then investigated.

Statistical Analysis
Continuous data were expressed as the median and interquartile range (IQR), whereas categorical variables were presented as counts or percentages.Univariate analyses (carried out by means of the Fisher's exact test or the chi-squared test in cases of categorical variables, or by means of the Mann-Whitney U test in cases of continuous variables) were performed for comparing the potential correlation of the patients' variables in attaining optimal vs. quasi-optimal/suboptimal piperacillin-tazobactam joint PK/PD targets, and in achieving microbiological eradication vs. microbiological failure.Statistical analyses were performed by means of MedCalc for Windows (MedCalc statistical software Ltd., version 19.6.1,Ostend, Belgium), and significance was defined as a p value < 0.05.

Results
Overall, a total of 35 hospitalized patients received definitive monotherapy with TDMguided CI piperacillin-tazobactam for treating secondary ESBL-producing Enterobacterales BSI during the study period.Demographics and clinical features of the included patients are reported in Table 2.The median (IQR) age was 79 years (68-85 years), with a slight male preponderance (51.4%).The median (IQR) CCI was 6 points (5-8.5 points), and 42.9% of cases were immunodepressed.Most patients were admitted to medical wards (21/35; 60.0%), and 10 (28.6%) were admitted to ICUs.
The vast majority of secondary BSIs were related to UTIs (27/35; 77.2%); four were related to IAI, and four were related to CR-BSI (11.4%) as the primary source.Most patients (30/35; 85.7%) had effective source control.Escherichia coli and Klebsiella pneumoniae were the only two species of Enterobacterales isolated, and were detected in 22 and 13 of the index blood cultures, respectively.Most isolates exhibited an MIC value of 8 mg/L, namely, borderline with the EUCAST clinical breakpoint (25/35; 71.4%).
Univariate analysis assessing variables potentially associated with microbiological eradication vs. failure is summarized in Table 3.Only quasi-optimal/suboptimal joint PK/PD target attainment of piperacillin-tazobactam showed a trend toward a higher risk of microbiological failure compared with optimal joint PK/PD target attainment (33.3% vs. 0.0%; p = 0.08; Figure 1).Relationship between microbiological outcome and optimal (green box), quasi-optim (yellow box) or suboptimal (red box) joint PK/PD target attainment of piperacillin-tazobactam. stronger trend toward higher microbiological failure rates was found among patients attaining t quasi-optimal/suboptimal joint PK/PD target of piperacillin-tazobactam than in those attaining t optimal target (33.3% vs. 0.0%; p = 0.08).CR-BSI: catheter-related bloodstream infection; IAI: intra dominal infection; UTI: urinary tract infection.

Discussion
To the best of our knowledge, this is the first prospective study that has explored th relationship between a joint PK/PD target attainment of CI piperacillin-tazobactam an the microbiological outcome among hospitalized patients receiving definitive CI piper cillin-tazobactam monotherapy for treating non-severe ESBL-producing Enterobacteral secondary BSIs.Notably, the findings showed that in the vast majority of patients, rea time TDM-guided ECPA programs of CI piperacillin-tazobactam facilitated optimal joi PK/PD target attainment and microbiological eradication.
In a recent retrospective study carried out among 43 ICU critically ill patients havin documented Gram-negative BSI and/or ventilator-associated pneumonia, we showed th the TDM-guided attainment of optimal joint PK/PD target of CI piperacillin-tazobacta monotherapy granted very high microbiological eradication rates (87.4%) and resulted protection against microbiological failure (OR 0.03; 95%CI 0.003-0.27;p = 0.002) [53].A hough in that study the number of patients having ESBL-producing Enterobacterales infe tions was quite limited (only 6/43), the findings allowed us to hypothesize that this stra egy could have been potentially effective even when dealing with ESBL producers [53].
Indeed, using definitive piperacillin-tazobactam monotherapy for treating ESB producing Enterobacterales BSIs was called into question by the findings of the MERIN trial, showing that, in this setting, piperacillin-tazobactam use was associated with high mortality rates compared with meropenem use [6].However, arguments that the resul of the MERINO trial could have been at least partially affected by the fact that PK/P target attainment of piperacillin-tazobactam could have been suboptimal due to interm tent infusion administration should not be overlooked [10].
Consequently, in the post-MERINO trial era, some guidance and/or viewpoin started suggesting that piperacillin-tazobactam administered by extended-infusion o A stronger trend toward higher microbiological failure rates was found among patients attaining the quasi-optimal/suboptimal joint PK/PD target of piperacillin-tazobactam than in those attaining the optimal target (33.3% vs. 0.0%; p = 0.08).CR-BSI: catheter-related bloodstream infection; IAI: intrabdominal infection; UTI: urinary tract infection.

Discussion
To the best of our knowledge, this is the first prospective study that has explored the relationship between a joint PK/PD target attainment of CI piperacillin-tazobactam and the microbiological outcome among hospitalized patients receiving definitive CI piperacillin-tazobactam monotherapy for treating non-severe ESBL-producing Enterobacterales secondary BSIs.Notably, the findings showed that in the vast majority of patients, real-time TDM-guided ECPA programs of CI piperacillin-tazobactam facilitated optimal joint PK/PD target attainment and microbiological eradication.
In a recent retrospective study carried out among 43 ICU critically ill patients having documented Gram-negative BSI and/or ventilator-associated pneumonia, we showed that the TDM-guided attainment of optimal joint PK/PD target of CI piperacillin-tazobactam monotherapy granted very high microbiological eradication rates (87.4%) and resulted in protection against microbiological failure (OR 0.03; 95%CI 0.003-0.27;p = 0.002) [53].Although in that study the number of patients having ESBL-producing Enterobacterales infections was quite limited (only 6/43), the findings allowed us to hypothesize that this strategy could have been potentially effective even when dealing with ESBL producers [53].
Indeed, using definitive piperacillin-tazobactam monotherapy for treating ESBLproducing Enterobacterales BSIs was called into question by the findings of the MERINO trial, showing that, in this setting, piperacillin-tazobactam use was associated with higher mortality rates compared with meropenem use [6].However, arguments that the results of the MERINO trial could have been at least partially affected by the fact that PK/PD target attainment of piperacillin-tazobactam could have been suboptimal due to intermittent infusion administration should not be overlooked [10].
Consequently, in the post-MERINO trial era, some guidance and/or viewpoints started suggesting that piperacillin-tazobactam administered by extended-infusion or, even better, by CI, could have represented a valuable option for treating ESBL-producing Enterobacterales secondary BSIs, especially whenever originating from sources at low-intermediate infection risk, namely, UTIs, IAIs, or CR-BSIs, being non-severe, and being caused by fully susceptible piperacillin-tazobactam strains [10,11,13,54].
The findings of our study may support the contention that administering piperacillintazobactam by CI and optimizing the joint PK/PD target attainment in real time thanks to a TDM-guided ECPA program may result in very high microbiological eradication rates among patients affected by non-severe secondary BSIs, even when caused by ESBLproducing Enterobacterales.Notably, in our study, we have introduced the innovative concept of joint PK/PD target for optimizing piperacillin-tazobactam therapy.According to this, in order to maximize clinical efficacy and prevent microbiological failure, it is important to attain an optimal PK/PD target not only for piperacillin, namely, the beta-lactam, but also for tazobactam, namely, the beta-lactamase inhibitor.In this regard, administering piperacillin-tazobactam by CI and adapting dosing regimens based on a TDM-guided approach may both maximize the PK/PD target of piperacillin to 100%f T >4-8×MIC and steadily maintain the tazobactam C ss above the fixed tazobactam target concentration of 4 mg/L proposed by the EUCAST for testing the in vitro susceptibility of the piperacillin-tazobactam combination [55].In the scenario of challenging clinical conditions, such as those of BSIs due to ESBL-producing Enterobacterales, attaining an optimal PK/PD target for both piperacillin and tazobactam could represent a major driver for improving both the clinical efficacy and prevention of Gram-negative resistance occurrence as much as possible, considering that different preclinical studies have reported a consistent decrease in piperacillin MIC values in the presence of a tazobactam concentration increase [56][57][58].Particularly, in a hollow-fiber infection model in which different ESBL-producing clinical isolates were tested, the attainment of a piperacillin-tazobactam exposure of %f T > instantaneous MIC (MIC i ; namely, the changing pathogen susceptibility in the presence of changing inhibitor concentrations) higher than 55.1-73.6% was significantly associated with the prevention of bacterial regrowth [56][57][58].Indeed, the confirmatory findings of this being a very suitable subset of patients in which applying this approach could represent a valuable carbapenem-sparing option, enabling improved antimicrobial stewardship programs focused at decreasing carbapenem use in settings with a high prevalence of carbapenemase-producing Gramnegatives [10].Obviously, appropriate source control should be mandatory in this context for minimizing either the risk of microbiological failure or that of relapse occurrence, as previously reported [54,59].
Finally, it should also be mentioned that the availability of a real-time TDM-guided ECPA program may prove to be extremely helpful in promptly recognizing and correcting cases having only quasi-optimal/suboptimal joint PK/PD target attainment of CI piperacillin-tazobactam. Indeed, the implementation of a real-time TDM-guided strategy was significantly associated with higher target attainment rates compared with the standard approach [60].
Limitations of our study should be acknowledged.The study design was monocentric, and the sample size was quite limited.Conversely, the prospective design is a point of strength, as was enrolling patients receiving piperacillin-tazobactam monotherapy.This confirmed the valuable role of piperacillin-tazobactam as a carbapenem-sparing strategy in this setting by avoiding any confounding bias on clinical and microbiological outcomes associated with the eventual use of combination therapy with other anti-Gram-negative antibiotics.

Conclusions
The preliminary findings of this prospective study suggest that real-time TDM-guided optimal joint PK/PD target attainment of CI piperacillin-tazobactam monotherapy may represent a valuable and effective carbapenem-sparing strategy when dealing with nonsevere ESBL-producing Enterobacterales secondary BSIs.Larger definitive confirmatory studies are warranted.

Table 1 .
Summary of the inclusion criteria for prospectively treating monotherapy patients having secondary bloodstream infections caused by ESBL-producing Enterobacterales with piperacillin-tazobactam.

Table 2 .
Demographic, clinical characteristics, and piperacillin-tazobactam treatment features of the included patients having definitive TDM-guided CI piperacillin-tazobactam monotherapy for treating BSIs caused by ESBL-producing Enterobacterales.

Table 3 .
Univariate analysis comparing patients' variables potentially associated with microbiological eradication vs. microbiological failure.